Say you have a multibyte data value and you are performing a read operation on it which is not atomic. Since the read operation is not atomic, there can be preemption between the assembly instructions of the read operation. After reading the first Byte, say there is a preemption and write operation starts execution. Since writes are atomic, it will completely write the multibyte value. Once the read operation resumes it's execution after write, it will read the next byte which is already modified by the previous write operation. This will lead to inconsistency.